The separation of sewage solids from water remains a fundamental issue in sewage treatment. Fundamentally, two physical processes can be used to remove solids from effluent: filtration and sedimentation.
A sewage stream is typically filtered by passing it through a porous medium. Either due to the surface active properties of the filter medium or the size of the solid material relative to the filter apertures, solid material is captured. Screening is a filtration process that relies solely on the relative size of the apertures and incoming solid material. The primary disadvantage of the filtration process is that the medium becomes fouled with time and must be cleaned. To achieve high removal rates for fine particles, back-flushing with complex process monitoring and control is required to ensure that filtration is effective.
Sedimentation is a process in which gravitational forces are used to separate sewage solids by virtue of their slight difference in density from water. The low relative density of the solids and the fine size of the constituent material have meant that the sedimentation process must take place in either very large quiescent flow-through tanks or by centrifuging in chambers to enhance the gravitational process. In general, centrifuging is not a continuous process or is restricted to very low processing rates.